Titanium has excellent corrosion resistance and high ratio of strength/specific gravity with no change in color and the like with the elapse of time. Therefore, titanium is expected to be useful as a material for products such as ornaments which a person puts on the body. Especially, in recent years, ornaments are required to be made of biocompatible material which does not cause metal allergy for human body. As titanium is a typical material which does not cause metal allergy, much attention has been focused on titanium as a material for ornaments. Under such circumstances, titanium has been superseding conventional metals such as stainless steel as a material for ornaments.
Ornaments are required to satisfy the requirements such as beauty in appearance, ability of being formed in a complicated and precise shape, and high resistance to getting flawed during the use in daily life. In addition, ornaments are required to have a clear mirror-finished surface which is capable of being formed with various kinds of surface finishings (such as letters written in extremely thin thickness to the extent of hair: hereinafter, referred to as hair-line property). From the viewpoint of machinability, ornaments are required to be produced with a number of minute holes satisfactorily formed therein.
In reality, titanium and titanium alloys used as a material for ornaments, and a method for producing ornaments are derived from the technologies developed in other industrial fields such as aerospace field, chemical industries, and nuclear energy field. Thus-produced ornaments do not necessarily satisfy the required properties.
Industrial pure titanium of JIS-1 series or JIS-2 series is one of the materials which are most commonly used for ornaments. However, the ornaments made of such titanium are easy to get flawed by contact or friction in a daily life, or their surface finishes are worn out. Therefore, the ornaments made of such titanium are inferior to those made of stainless steel in beauty and decorativeness, which are essential properties for ornaments.
As an titanium alloy includes large amount of alloy elements, ornaments made of titanium alloy have increased strength and high resistance to flaw. In this respect, an titanium alloy is superior to industrial pure titanium as a material for ornaments. However, as an titanium alloy is poor in processability, it is difficult to conduct a precise and minute machining thereto, resulting in imposing limitations on designs of ornaments. In addition, almost all titanium alloys include alloy elements such as aluminum, nickel, vanadium, chromium. These alloy elements are poor in biocompatibility, and therefore, not desirable for ornaments. In addition, as these alloy elements are relatively expensive, the material cost becomes high.
Various technologies have been developed for improving the abrasion resistance of industrial pure titanium and the machinability of titanium alloy. However, these technologies are intended for use in the fields other than ornament industries, and therefore, are not applicable to ornaments. For example, Japanese Patent publication No. 7-62196 suggests a titanium alloy having an improved abrasion resistance. In this art, the abrasion resistance of the titanium alloy is improved by dispersing titanium carbide. However, the obtained titanium alloy has too high hardness, resulting in shortening a life of drill which is used in a drilling process of forming minute holes for ornaments. Japanese Patent publication No. 5-42490 suggests a titanium alloy in which inclusions such as sulfide are dispersed for the purpose of enhancing the machinability and free-cutting properties thereof. However, the inclusions are too soft to give the produced ornaments an enhanced resistance to flaw. Rather, the presence of such inclusions large in size may be a hindrance to give a mirror-finished surface to ornaments.
On the other hand, conventional methods do not necessarily contribute to the improvement of the properties of ornaments. For example, Japanese Laid-Open patent publication No. 3-180478 suggests technology where a surface of pure titanium material is coated with a hard coating, thereby enhancing its resistance to flaw. However, the surface treatment of this technology has a problem that the coated surface loses metallic luster or turns into darker color, resulting in the deterioration of beauty. In addition, as titanium used as a parent material itself is easy to get flawed, the titanium material may be flawed during the processes conducted prior to a surface treatment. The resultant ornaments having flaws are of no commercial value.
In order to enhance the strength of products, there is a known method in which a thermal treatment is conducted. When the thermal treatment is conducted, however, the product gains high hardness not only on the surface thereof but also at the inside thereof. As a result, the whole product gains high hardness and satisfactory machining cannot be conducted thereto. In addition, the thermal treatment has a hardness-increasing effect only on a .beta.-type titanium alloy or an .alpha.+.beta.-type titanium alloy which contain large amount of alloy elements. Although a cold working such as a cold forging induces a work hardening thereby attaining the increase in hardness, a cold forging increases the hardness of the whole product and the machinability remains unimproved. In contrast, a method such as shot peening is capable of increasing the hardness of the product on only the surface thereof by forming a strained region only on the surface. However, she shot peening is not applicable to the production of products required to be formed in a refined shape.
In the actual state, in the production of ornaments made of pure titanium, industrial pure titanium material having low resistance to flaw is used without being subjected to any treatment, or is surface treated at the sacrifice of decorativeness. There are some cases where a Ti-3Al-2.5V type titanium alloy is used as a material for ornaments. The Ti-3Al-2.5V type titanium alloy has an intermediate characteristics between industrial pure titanium and the above-described titanium alloy. However, the Ti-3Al-2.5V type titanium alloy does not satisfy the requirement of the resistance to flaw, processability, and cost performance, and contains unbiocompatible elements such as aluminum and vanadium. Due to such disadvantages, the Ti-3Al-2.5V type titanium alloy can be used only a limited application.
As described above, the conventional titanium, titanium alloys, and method for producing ornaments using these materials are not suitable for ornaments. Under such circumstances, there is a great demand for developing titanium material and a method for producing products using the titanium material capable of attaining excellent properties such as decorativeness, durability, processability, biocompatibility with low cost, thereby coming such material and method into wide applications including not only ornaments but also decorations and articles of daily use.
The present invention has been conducted to solve the above-described problems, and the objective thereof is to provide: a high strength titanium alloy capable of attaining beauty and decorativeness and especially useful as a material for ornaments; products made of such titanium alloy; and a method for producing the products.